Flow completion systems typically include a wellhead which is positioned at an upper end of the wellbore and a tubing hanger which is landed in the wellhead or in a christmas tree that is mounted to the top of the wellhead. In such systems the wellhead and the christmas tree together with the tubing hanger form a pressure-containing barrier between the wellbore and the surrounding environment. This pressure barrier must be maintained at all times during operation of the flow completion system in order to prevent well fluids from leaking into the surrounding environment.
Flow completion systems usually include a number of downhole devices which need to be accessed from an exterior location. For example, a monitoring and control system located, e.g., on a surface vessel commonly receives inputs from a number of downhole sensors. The downhole sensors are typically connected to corresponding downhole data and/or power cables. In order to provide for communication between the monitoring and control system and the downhole sensors, the downhole data and/or power cables must normally be connected to corresponding external data and/or power cables which in turn are connected to the monitoring and control system.
One way of connecting the downhole cables to their corresponding external cables is through the use of a downhole feedthrough system. A typical downhole feedthrough system includes a penetrator which is mounted on the wellhead or christmas tree. One end of the penetrator is connected to the external data and/or power cables and the other end extends through a feedthrough port in the christmas tree or wellhead and engages a connector which is mounted in the tubing hanger. The connector in turn is connected to a number of data and/or power cables which are positioned in axial feedthrough bores in the tubing hanger and are connected to the downhole data and/or power cables by additional connectors.
However, this type of arrangement is undesirable for several reasons. First, the feedthrough port in the christmas tree or wellhead and the feedthrough bore in the tubing hanger denigrate the critical pressure barriers provided by these components. Second, in order to seal the potential leak path posed by the feedthrough port in the christmas tree or wellhead, the penetrator must be provided with several robust sealing systems, and this complicates the design and increases the cost of the penetrator. Third, the relatively large size of the penetrator limits the number of penetrators which may be incorporated into a typical flow completion system, and this in turn limits the number of downhole lines which can be employed in the system. Fourth, since tubing hangers typically have limited space available for feedthrough bores, the number of downhole lines which can be accessed through the tubing hanger is restricted.
Present day flow completion systems typically must be designed with the ability to measure various wellbore parameters such as temperature, pressure and flow in order to provide the operator with an understanding of the conditions in the wellbore and the reservoir. Although many sensor types are available for such measurements, the harsh wellbore environment prohibits the use of off-the-shelf devices. The operating environment for wellbore sensors may include temperatures of up to 300° C. and pressures of up to 15,000 psi, as well as a variety of production fluids, which are often loaded with abrasive sand and rock fragments. Until recently, wellbore measurements were largely performed using specially constructed electronic sensors. Although many of these devices are highly sensitive and accurate, the harsh wellbore conditions, particularly the elevated temperatures, can reduce their operational lifetime or restrict their use. The elevated temperatures can also cause problems in communicating with the sensors using electrical cables. Consequently, only a relatively small number of electronic sensors are typically deployed, thus limiting the type and amount of information that may be provided.
One solution to this problem has been to employ fiber optic sensors to measure wellbore parameters. Optical fiber sensor and communication systems are much more compatible with the downhole environment. Optical fiber sensors offer the ability to provide both point and distributed wellbore sensing systems which are capable of generating the real time data required for effective optimization of the hydrocarbon production process. A number of optical fiber point sensors have been developed for wellbore sensing applications, examples of which include Bragg grating-based temperature, pressure, strain and flow measurement sensors. Such sensors may, for example, be used to monitor temperature at discrete locations, the strain on a well casing and the position of a sliding sleeve valve. Examples of optical fiber distributed sensors include those which use Raman scattering for measuring temperature and Brillouin scattering for measuring temperature and strain. Such measurements may be used to determine the temperature profile of a well and may, for example, provide real-time assessment of inflow or injection distribution.
An example of a prior art downhole feedthrough system for a fiber optic sensor system is shown schematically in FIG. 1. The downhole feedthrough system is shown installed on a flow completion system 10 which comprises a christmas tree 12 located at the top of a wellbore, a tubing hanger 14 landed in the christmas tree and a tubing string 16 connected to the bottom of the tubing hanger. The optical downhole feedthrough system provides for communication of optical signals between one or more downhole fiber optic sensing device 18 and an external fiber optic cable 20 which is connected to a monitoring and control system located, for example, on a surface vessel (not shown). The optical downhole feedthrough system includes a penetrator assembly 22 which is mounted to the outer surface of the christmas tree 12. The penetrator 22 includes a fiber optic cable 24 having a first end which is connected to the external cable 20 via a conventional dry mate connector 26 and a second end which is connected to a first wet mate connector 28. The first wet mate connector 28 is supported on a movable stem 30 which when the penetrator 22 is actuated moves the first wet mate connector through a feedthrough port 32 in the christmas tree 12 and into connection with a second wet mate connector 34 mounted in the tubing hanger 14. The second wet mate connector 34 is connected to a fiber optic cable 36 which is positioned in an axial feedthrough bore 38 in the tubing hanger 14 and is connected via a pair of dry mate connectors 40, 42 to a downhole fiber optic cable 44 that in turn is connected to the downhole device 18.
Although the optical downhole feedthrough system shown in FIG. 1 provides a means for establishing communications between an external monitoring and control system and a number of downhole fiber optic sensors, this system nevertheless suffers from the same disadvantages as the electrical cable-based system described above. In particular, because the feedthrough system requires a feedthrough port in the christmas tree, the pressure barrier provided by the christmas tree must be breached and the penetrator must be designed to include robust sealing systems for containing the wellbore pressure.
An embodiment of an optical downhole feedthrough system which does not require a penetration through the pressure barrier is discussed in U.S. Pat. No. 7,845,404, which is hereby incorporated herein by reference. In this embodiment, an optical downhole feedthrough device which is mounted to a christmas tree comprises an optically transparent window and optical repeaters positioned on either side of the window. The window and optical repeaters allow optical signal to be communicated between entities located inside and outside the christmas tree without penetrating the pressure barrier.